Air pollution occurs when undesirable gases or particles are introduced into the air that humans breathe. Common pollutants include carbon monoxide, sulfur dioxide, nitrogen oxides, ground-level ozone, lead, sand, dust particles, and other particulate matter. In large cities around the world, man-made air pollution and pollution related ailments tend to increase with population density. Common pollution related ailments include respiratory diseases, cardiovascular diseases, cancers, adverse pregnancy outcomes, and even death. Conventional pollution masks seek to protect the wearer from inhaling certain gases or particles in polluted areas.